Machinery with high speed rotating components that experience a wide range of environmental conditions, such as miniature gas turbine or turbojet engines, require bearing systems for such rotating components that achieve reliable operation with minimal cost, particularly since many such miniature engines are of the single-use expendable type.
The bearing system in such miniature gas turbine engines usually supports a rotor shaft for the rotating components. It comprises an aft rotary bearing to support an aft journal for the rotating components and at least one forward rotary bearing to support a corresponding shaft forward section. A housing for the engine generally attaches to an outer race of the aft bearing to hold it in a fixed axial and radial position. To compensate for any change in length between the shaft aft section and the shaft forward section due to changes in environmental conditions, such as ambient or operating temperatures, the housing fixes the radial position of an outer race for the forward bearing but allows axial freedom of movement of the outer race to allow the bearing system to compensate for such changes in axial length between the shaft aft section and the shaft forward section. This allows the bearing system to support the rotating components without bearing seizure due to environmental changes in axial length between the shaft aft section and the shaft forward section. In other words, the bearing system includes a linear bearing between the housing and the outer race of the forward bearing.
It is important that this linear bearing does not itself seize up before or during operation of the engine. It is possible to lubricate this bearing with the engine lubrication system. However, the environmental condition for the engine before starting may be so extreme, such as at a very low ambient temperature, that due to differences in the coefficient of expansion between the housing material and the material of the forward bearing outer race, the clearance between their mating surfaces may diminish to the point that they bind or seize together. In this case, any sort of lubrication provided by the engine lubrication system is useless to free up these surfaces, and the aft and forward bearings may then exhibit a degree of axial tension that causes them severe wear or seizure that in turn may cause premature engine failure.